<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>7固定定位</title>
  <style>

    .fu{
      background-color: #ff0;
      width: 500px;

    }
    .fu>div{
      width: 100px;
      height: 100px;
    }
    .z1 {
      background-color: #f00;
    }
    .z2 {
      background-color: #0f0;
        /*
            固定定位 参照物为html元素 会脱离文档流释放布局空间
        */
        position: fixed;
        left: 0;
        /*top: 0;*/
        bottom: 0;
    }
    .z3 {
      background-color: #00f;
    }
  </style>
</head>
<body>
<div class="fu">
  <div class="z1"></div>
  <div class="z2"></div>
  <div class="z3"></div>
</div>
</body>
</html>